Time to rethink well-abandonment techniques

In using cement to create an annular sheath to seal steel drilling pipe to a productive hydrocarbon formation, the industry has been making a subtle but serious mistake.

In many applications steel and cement are marvellously adaptable materials and work together superbly. Unfortunately, that is not the case in oil and gas wells.

Steel is dense, has a very high tensile strength, is elastic, and has an exceptionally long working life if protected from corrosion. Shaped into pipe, it can handle very high pressures at extremely high temperatures. Without a doubt steel pipe is the best material for the containment and transmission of oil and gas liquids from the reservoir to the processing facilities.

Cement, by comparison, has very little tensile strength, only modest compressive strength, and virtually no elasticity. It has the value of being miscible with water; the resulting mixture can be pumped as a liquid or slurry and will then set as an impermeable solid sheath around steel well pipe.

But the major deficiency of cement as a well annular or bore sealant is that continuous dehydration causes it to shrink volumetrically by 0.5% to 1% over 15 to 20 years. This shrinkage often creates micro-annular leak paths between the outside diameter of the pipe and the cement, the outer edge of the cement and the reservoir rock, or through cracks in the cement itself.

The efficacy of cement as a well sealant is also dependant on proper placement techniques, which may not always have been followed in the haste to bring oil or gas production online. Poor hole clean-up to remove drilling mud “cake,” highly permeable “thief zones” into which the cement just disappears, and poor cement blending have all created less than ideal hydraulic cement seals in many wells.

In fairness to our petroleum engineering predecessors, the shortcomings in well design were not obvious until late in the last century with the rapid increase in the number of gas wells drilled in the 1980s through the early 2000s. It is now evident that the micro-annular paths facilitate gas leakage to the surface in any well with a significant differential pressure between the surface and producing (or cemented-off, non-commercial gas formation(s).

This leakage is known as surface casing vent flow (SCVF). In western Canada, the incidence of SCVF in inactive wells is approaching 20%. In previously plugged and abandoned wells the leak rate is near 9%. Regulatory changes in Alberta in the late 1990s allowed operators to suspend gas wells for an indeterminate time. This has resulted in the accumulation of nearly 82,000 “inactive” wells in the province. As of 2016, some 17,000 of these inactive wells are known to have SCVF.

The remediation of SCVF is a very contentious issue and the methods to deal with it are imprecise. There is often no consensus on the cause, and any given well may have numerous separate sources of gas contributing to SCVF. Sealing off these various sources is difficult and expensive, and not always effective; the success rate of the first attempts at sealing SCVF leaks is less than 25%.

Locating the source of a SCVF is also hit and miss. The standard technique is to analyze data from logging performed during drilling-and-completion operations. Such data provides information about the geology and productive capability of the reservoir. A wide range of logging techniques have been developed since Schlumberger pioneered the first electro-magnetic (EM) logs in the 1950s. Logs commonly show a characteristic known as “cement bond.” A cement-bond log (CBL) is a rudimentary evaluation of the steel-to-cement interface. A poor CBL often indicates a potential SCVF pathway.

Contributing to the problem of leaking wells in Alberta is that the Alberta Energy Regulator (AER) strictly prescribes an abandonment process that incorporates cement. In the absence of better alternatives, the AER also mandates that wells with faulty cement seals be repaired with more cement.

The flawed repair/abandonment process relies on the weakest component of well design: cement.

The industry standard technique for the internal plugging of leaky wells is to place cement sealant on top of a bridge plug. In damaged permeable cement sheaths the practice is to inject cement through perforations (“cement squeeze”). With both procedures it is common to see a recurrence of leakage after 15 to 20 years as the cement ages and shrinks.

Since the problem is cement, why not eliminate it entirely and take advantage of the elastic properties of steel to create a permanent seal? Winterhawk has designed an alternate technology that addresses the problems created by cement in abandonments and in SCVF remediation.

The Winterhawk technology uses a combination of mechanical and thermal forces to expand the dynamic steel well casing radially against any existing cement sheath. The mechanical device—a series of stacked, expanding stainless-steel rings—provides sufficient force to expand the well casing and close off micro-annular leak paths. The thermal component is polymer-modified asphalt (PMA), which, used in conjunction with the tool, forms a plug inside the pipe. Material analysis on stainless steel and asphalt projects the technology will have a life expectancy of 300 years or more.

In instances where the cement sheath is damaged beyond repair (e.g., worm holes; large cracks), the liquid PMA can be used as a sealant and injected (“squeezed”) through abrasa-jet cuts in the casing. Access to the casing cement annulus is created with one or more partial circumferential slots through which the PMA may be injected. The PMA follows the micro-annular pathways through capillary action.

The viscosity of the PMA is less than that of cement slurry, yet it is extremely tenacious. When set, it will create a robust bond with the steel casing and the reservoir rock. The PMA sets in less than 20 minutes and does not require high-injection pressures or flow rates.

By replacing cement with a relatively low-cost alternate sealant reinforced by an expanding stainless-steel labyrinth seal, Winterhawk’s technology will provide secure abandonments and offer a high-integrity, long-life means to remediate SCVF wells.